The present invention pertains to a heat recording method, such as heat sensitive recording, heat transcription recording, conduction heat sensitive recording, conduction transcription recording, thermal ink jet printing and the like. More particularly, the present invention pertains to a driving method for driving heat generating resistors in a heat recording device.
Conventional heat recording methods are known in which heat generated by heat generating resistors of a thermal head is transmitted directly to a heat sensitive paper and the like. Also, in a thermal ink jet system, bubbles are produced by heat generated from heat generating resistors of a thermal head. These bubbles create a pressure which results in the liquid ink jet used for printing. In these conventional heat recording methods, metal compound resistors made of ruthenium oxide, tantalum nitride are known. Also, such thermal resistors may include an insulating material having a high melting point such as silicon oxide and tantalum.
When a voltage is applied to the conventional heat generating resistors of the conventional thermal head, electric current passes through the heat: generating resistor to generate Joule heat. By maintaining this voltage for a predetermined time, the heat energy necessary for recording is applied to the recording medium. The Joule heat energy generated in the conventional heat generating resistor is determined by the resistance value of the resistor, the applied voltage, and the time duration that the voltage is applied. Conventionally, the characteristics of the conventional heat generating resistor have been controlled depending on various factors, such as the characteristics of the heat sensitive paper used, heat transmitting characteristics between the resistor and the paper, background temperature, the temperature of the recording medium, etc. The applied voltage or the duration of the voltage application is conventionally regulated in order to obtain the most suitable recording quality.
In another conventional recording method, known as the electric conduction transcription recording method, an ink donor sheet or the like is used which has an electric conduction heat generating resistance layer which may be made of a carbon paint. An electric conduction head is used to pass current to the electric conduction heat generating resistance layer which heats the ink on the ink donor sheet causing it to heat or sublime so that it can be applied to a recording medium. As with the other conventional recording methods, it is desirable to optimize the printing obtained by this method. Thus, the characteristics of the electric conduction heat generating resistance layer are controlled to optimize the printing results.
In the conventional heat recording methods, the printing results are sought to be optimized by controlling the heat energy of the recording device by adjusting the applied voltage and the voltage applying pulse width. However, controlling the applied voltage and voltage applying pulse width is extremely difficult resulting in recording instruments which are large and expensive.
The Joule heat energy generated by the voltage pulse applied to the heat generating resistor can be controlled by controlling the voltage or controlling the pulse width. However, the temperature obtained by the heat generating resistor is inconsistent and changes due to the application period of the voltage applying pulses, the number of continuously applied pulses, the proximity of other heat generating resistors, the temperature of the supporting substrate of the thermal head, the temperature of the ink donor sheet and the liquid ink, ambient temperature and other factors.
The magnitude of the heat energy generated by the heat generating resistor depends on the temperature of a color generating layer in the heat sensitive paper and the temperature of the ink layer. Also, it depends on the temperature of the heat generating resistor. Thus, in order to obtain a uniformly recorded heat recording, it is desirable to provide a temperature which is uniform. To make the temperature uniform it must first be determined what adjustment of the voltage or of the voltage application pulse width must be made so that the heat generated by the heat generating resistor is consistent and rises to a specified temperature. Thus, thermal environmental information must be collected or assumed and thermal history information of the heat generating resistor must be determined.
Such information collecting means, assuming means, and recording condition determining means are extremely expensive and require various kinds of temperature sensors for detecting the temperature of the thermal head substrate and ambient temperature. Also, memories for recording the recorded data of the heat generating resistor history, simulators such as a CPU for effecting arithmetic treatment, gate circuits, etc. are required. Furthermore, extremely complicated software must be utilized to obtain meaningful results. In particular, when a large sized high precision heat recording device which has a large number of heat generating resistors is used, such information collecting means, assuming means and recording condition determining means become extremely expensive and often times the recording quality is sacrificed. Furthermore, the time required to collect and determine the information is restricted by the CPU and has become an obstacle to high speed recording.
A glaze layer has been used on a thermal head as a temperature preserving layer for enhancing heat efficiency in general. However, this glaze layer is made by a thick film process and the fluctuation of the thickness reaches more than plus or minus 20% of an average value. Thus, the heat preserving effect of this glaze layer in each individual thermal head fluctuates greatly. Thus, the information collecting means, assuming means and record condition determining means used for heat generation temperature control at a high precision cannot be realized because of the fluctuation of characteristics of each individual thermal head. The fluctuation of thermal characteristics of each individual thermal head must be taken into consideration as a control parameter, which greatly sacrifices the ability of mass producing such thermal heads. Furthermore, the interchangeability of the thermal head of a recording instrument is sacrificed because of this need to adjust the individual characteristics of each thermal head. In the case of current passing heat recording, the same fluctuation of the heat capacity and heat resistance exists in the circumferential part of the heat generating resistance layer. Thus, there are the same problems as described above with regard to thermal heads.